New wireless communications standards such as for example Wireless Fidelity (WiFi), Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE), uses Orthogonal Frequency Division Multiplex (OFDM) based modulation schemes. These methods are substantially different from Third Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) which uses Code Division Multiple Access (CDMA), and consequently the computing elements in implementations of these standards are different. This makes it difficult to implement a device that support both CDMA based standards and OFDM based standards.
In CDMA, a commonly used receiver algorithm is the “rake receiver”. It is fundamentally based on a number of parallel receiver structures, often called “rake fingers”. Each “rake finger” is aligned in time with a peak in the impulse response of the transmission channel (which in general is a multi-path channel, i.e. a fading channel). For complex channels, e.g. multiple transmission paths, and in combination with Doppler shifts many rake fingers must be used, and often must be re-aligned as transmission paths fade away, and new appears. To handle these scenarios it is desired to assign a greater number of “rake fingers” to each “rake finger”. Thus, the total number of “rake fingers” is a major limiting factor for the capacity of a rake receiver device.
A standard WCDMA rake receiver front-end comprises a few major operations: searching, de-spreading, Radio Frequency (RF) channel coefficient estimation, and de-spread and Maximum Ratio Combining (MRC).
The searching operation is a correlation of the received signal with a known pattern. The outcome of this operation is a timing alignment of the received signal. It also gives information of the alignment of the individual multipath components, i.e. the delay of each individual propagation path from the distant transmitter to the receiver.
De-spreading (and descrambling) is then performed based on the delays obtained during the search operation. The outcome of this operation is crude radio frequency channel coefficient measurements, i.e. the amplitude and phase of each individual multipath component.
The channel coefficient estimation is most of all a filter operation. Filter parameters are adapted to match the fading frequency of each multipath component. The crude radio frequency channel measurements are thus filtered to remove as much measurement noise as possible. At this stage other algorithms may be applied to these coefficients as well.
De-spread and MRC is the final stage in the operations of a standard WCDMA rake receiver, where each individual multipath component is aligned in time (based on delays from the search operation), scaled according to the phase and amplitude of the radio frequency channel coefficients (from the previous channel coefficient estimation operation) and then added together based on the orthogonal WCDMA channelization code (this is the actual de-spreading function). The outcome is symbol estimates of the transmitted symbols (picked from a modulation constellation such as Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK)).
The symbol estimates are in later stages of the WCDMA rake receiver demodulated to soft bit estimates, and further processed according to the Forward Error Correction (FEC) scheme dictated by a standard.
There are of course many options in which order these operations are done. The possibility to permute different processing stages is typical for signal processing.
In a standard LTE/WiMAX receiver, timing advance is used. Searching operations are used to estimate the timing alignment of distant transmitters, and then instructed to change their timing, so that signals from distant transmitters are arriving with the same timing at the base station receiver.
The received signal is divided into intervals based on the OFDM symbol length, and transformed to the frequency domain using the Fast Fourier Transform (FFT) algorithm.
The operations of RF channel coefficient measurements, processing and the final MRC in a LTE/WiMAX receiver are in principle similar to the WCDMA processing, but made in the frequency domain.